This invention relates generally to expansion valves for refrigeration systems and particularly to an expansion valve having a dual port and pin valve assembly to control refrigerant flow through the valve and preferably uses a stepper motor actuator to operate the valve assembly.
In general, refrigeration systems of the type under consideration consist of a compressor, an evaporator, a condenser and an expansion device, typically a thermostatic expansion valve. The primary purpose of the expansion valve is to control liquid refrigerant flow to the evaporator so that a predetermined amount of superheat vapor is maintained in the suction line.
Conventional thermostatic expansion valves usually incorporate a motor element such as a diaphragm. The diaphragm is responsive to the heat in the system, for example at the evaporator outlet, and controls a valve port within the valve which is located between the valve inlet and the valve outlet. Conventionally, the diaphragm is connected to the valve pin by pushrods and, while this actuator system works well in many applications, there is a need for more precise flow control in other applications.
Theoretically, diaphragm actuated expansion valves have infinite positional control. However, factors such as diaphragm stresses, pushrod friction, and spring side loading create valve hysteresis. It is hysteresis that makes infinite positional control unachievable in practice. Also, diaphragm actuated expansion valves have difficulty providing the precision movement required when it is desired to lower superheat settings and insure stable control.
In addition, the stroke limitations of a diaphragm, and the fact that these valves are sized for maximum, or worst case, load conditions, drastically limit their ability to provide suitable performance under partial load conditions. Most refrigeration and air conditioning systems operate at less than maximum load most of the time. Therefore, an expansion device capable of providing lower stable superheat control at reduced loads would provide improve operating efficiency.
To meet this need, the control of the expansion valve by electronic rather than by the more direct thermostatic means has been used. There are advantages in using a stepper motor to control the valve flow as discussed in commonly owned U.S. Pat. No. 4,911,404, which is incorporated herein by reference. For example, a stepper motor actuated valve provides discrete segments of valve stroke for each incremental angular movement, or step. A stepper motor has the advantage of being readily incorporated into a digital control system because the reliability and low hysterisis provide precise, repeatable valve positions. In this way, the creation of a hunting or cyclic pattern, which has historically been associated with conventional expansion devices, is avoided.
A stepper motor actuated valve has unlimited stroke potential, but fixed positional control. The valve can be designed to provide stroke and positional control that will allow for flow characterization of the port. The right combination of stroke, positional control, and flow characterization can provide flow resolution better than 0.5.degree. F. superheat/step.
Probably the greatest advantage of a stepper motor actuated expansion valve is the capability of precise control of refrigerant flow through the evaporator. However, the capability of the valve to provide a high degree of flow control leads to other advantages, such as lower "stable" system superheat; lower return gas temperatures, and higher back pressures. These advantages can contribute to less dehydration of the refrigerated air space; more efficient use of the evaporator surface, and improved operating energy efficiency. Stepper motor actuated valves have other advantages such as being independent of system pressures; being direct acting thereby providing bidirectional flow, and having tight seating liquid line shut-off capabilities. Also, the fact that the stepper motor actuated valve is an electromechanical device allows for all of the advantages of a microprocessor control.
However, the use of a stepper motor is not a solution to all problems and attempts have been made to improve the valve port characteristics. It is to the improvement of valve port characteristics, particularly in conjunction with a stepper motor actuator, that the present invention is directed.